Re: [PATCH v2 3/3] dt-bindings: thermal: update sustainable-power with abstract scale

From: Doug Anderson <dianders@chromium.org>
To: Lukasz Luba <lukasz.luba@arm.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	linux-doc@vger.kernel.org,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>, Rob Herring <robh+dt@kernel.org>,
	amitk@kernel.org, Jonathan Corbet <corbet@lwn.net>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Dietmar.Eggemann@arm.com, Quentin Perret <qperret@google.com>,
	Matthias Kaehlcke <mka@chromium.org>,
	Rajendra Nayak <rnayak@codeaurora.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH v2 3/3] dt-bindings: thermal: update sustainable-power with abstract scale
Date: Fri, 2 Oct 2020 07:31:17 -0700	[thread overview]
Message-ID: <CAD=FV=UbNP5-G1z95F37Fmv8=n0JPSSwnPQO_K==WpAc4vAHWQ@mail.gmail.com> (raw)
In-Reply-To: <20201002114426.31277-4-lukasz.luba@arm.com>

Hi,

On Fri, Oct 2, 2020 at 4:45 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
>
> Update the documentation for the binding 'sustainable-power' and allow
> to provide values in an abstract scale. It is required when the cooling
> devices use an abstract scale for their power values.
>
> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
> ---
>  .../devicetree/bindings/thermal/thermal-zones.yaml  | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/thermal/thermal-zones.yaml b/Documentation/devicetree/bindings/thermal/thermal-zones.yaml
> index 3ec9cc87ec50..4d8f2e37d1e6 100644
> --- a/Documentation/devicetree/bindings/thermal/thermal-zones.yaml
> +++ b/Documentation/devicetree/bindings/thermal/thermal-zones.yaml
> @@ -99,10 +99,15 @@ patternProperties:
>        sustainable-power:
>          $ref: /schemas/types.yaml#/definitions/uint32
>          description:
> -          An estimate of the sustainable power (in mW) that this thermal zone
> -          can dissipate at the desired control temperature. For reference, the
> -          sustainable power of a 4-inch phone is typically 2000mW, while on a
> -          10-inch tablet is around 4500mW.
> +          An estimate of the sustainable power (in mW or in an abstract scale)
> +         that this thermal zone can dissipate at the desired control
> +         temperature. For reference, the sustainable power of a 4-inch phone
> +         is typically 2000mW, while on a 10-inch tablet is around 4500mW.
> +
> +         It is possible to express the sustainable power in an abstract
> +         scale. This is the case when the related cooling devices use also
> +         abstract scale to express their power usage. The scale must be
> +         consistent.

Two thoughts:

1. If we're going to allow "sustainable-power" to be in abstract
scale, why not allow "dynamic-power-coefficient" to be in abstract
scale too?  I assume that the whole reason against that originally was
the idea of device tree purity, but if we're allowing the abstract
scale here then there seems no reason not to allow it for
"dynamic-power-coefficient".

2. Is it worth adding some type of indication of what type of units
"sustainable-power" is represented in?  Maybe even a made up unit so
that you could tell the difference between made up units in the same
system?  I'd envision something like:

sustainable-power-units = "qualcomm,sc7180-bogoWatts"

...and on the dynamic-power-coefficient side, the same:

dynamic-power-coefficient-units = "qualcomm,sc7180-bogoWatts"

One could imagine someone even later (after devices are widely
distributed) figuring out translations between these bogoWatts numbers
and real Watts if someone could come up with a case where it matters.

-Doug